Add Brotli support

[erikdubbelboer]

New Functions:

    CompressHandlerBrotliLevel(h RequestHandler, brotliLevel, otherLevel int) RequestHandler
    Request.BodyUnbrotli() ([]byte, error)
    Response.BodyUnbrotli() ([]byte, error)
    AppendBrotliBytesLevel(dst, src []byte, level int) []byte
    WriteBrotliLevel(w io.Writer, p []byte, level int) (int, error)
    WriteBrotli(w io.Writer, p []byte) (int, error)
    AppendBrotliBytes(dst, src []byte) []byte
    WriteUnbrotli(w io.Writer, p []byte) (int, error)
    AppendUnbrotliBytes(dst, src []byte) ([]byte, error)

New Constants:

    CompressBrotliNoCompression
    CompressBrotliBestSpeed
    CompressBrotliBestCompression
    CompressBrotliDefaultCompression

Brotli compression levels are different from gzip/flate. Because of this we have separate level constants and CompressHandlerBrotliLevel takes 2 levels.

I didn't add Brotli support to CompressHandler as this could cause a spike in CPU usage when users upgrade fasthttp.

fasthttp.CompressBrotliDefaultCompression is not the same as
brotli.DefaultCompression. brotli.DefaultCompression is more than twice
as slow as fasthttp.CompressBrotliDefaultCompression which I thought was
unreasonable as default.

[kirillDanshin]

I tried to find a shorter, and cleaner option for this, but still can't find a better way.
what I see here is that now, you would have to write

h = fasthttp.CompressHandlerBrotliLevel(h, fasthttp.CompressBrotliDefaultCompression, fasthttp.CompressDefaultCompression)

or

h = fasthttp.CompressHandlerBrotliLevel(h, 4, 6)

first is barely readable, and the second isn't obvious without IDE reminding you the order. maybe, you have some ideas on that?

otherwise, this looks good to me

[erikdubbelboer]

How is the first not readable? It's very readable as it spells our exactly what it is. Yeah the line is a bit long but who works in an editor with 80 characters max with these days...
Or if you think it's too long you can just write it like

h = fasthttp.CompressHandlerBrotliLevel(
    h,
    fasthttp.CompressBrotliDefaultCompression,
    fasthttp.CompressDefaultCompression,
)

[proyb6]

I like to comment if the ”Compress...” is not need since it’s understood that Brotli is a compressed format or:

fasthttp.SetHandlerBrotliLevel(...)

[erikdubbelboer]

We have CompressHandler and CompressHandlerLevel. It would be super weird to call this similar method SetHandlerBrotliLevel all of sudden. It's better to keep names similar.